1. Field of the Invention
The present invention relates to a Karman vortex flow meter applicable to internal combustion engines of motor vehicles or the like for measuring flow velocity and flow rate of fluid, and more particularly to a system to detect a Karman vortex through the use of a hot wire or the like.
2. Description of the Related Art
As systems to detect a Karman vortex through the use of a hot wire, there have been known a method of directly detecting the Karman vortex occurring in a fluid through the use of a hot wire and a method of detecting a secondary flow due to alternating Karman vortexes. For example, the former is disclosed in Japanese Patent Laid-Open No. 57-86013 (see FIG. 11), while the latter is been disclosed in Japanese Patent Laid-Open No. 57-17864 (see FIG. 12) or Japanese Utility Model Laid-Open No. 5-84825 (see FIG. 13).
In FIG. 11, a vortex generating pole or column 2 is placed in a duct 1 through which a fluid flows, and hot wires 9a and 9b are symmetrically disposed in a vortex passing region existing on the downstream of the vortex generating pole 2. This structure detects a Karman vortex 7 generated alternately on the downstream of the vortex generating pole 2 by the flows of a fluid indicated by an arrow 10 as the increase and decrease in the heating current fed to the hot wires 9a and 9b, thereby measuring a flow velocity or flow rate on the basis of a period of generation of the Karman vortexes.
On the other hand, in FIG. 12, a hot wire 9 is stretched in a through hole 5 which establishes communication between both side portions of a vortex generating pole 2 located within a duct 1 in which a fluid flows, so that the flow produced in the through hole 5 following the generation of Karman vortexes 7 is converted utilizing the variation in the radiating quantity of the hot wire 9 into an alternating current signal synchronizing with the vortex generation to measure the flow velocity or flow rate on the basis of the frequency or period of this alternating current signal.
Furthermore, in FIG. 13, pressure measuring holes 11 are made in right and left side surfaces of a vortex generating pole 2 fitted in directions perpendicular to the direction of flow in a measurement passage, and these pressure measuring holes 11 are communicated with each other through a detection passage 12. In addition, a thermal flow sensor 13 is situated within the detection passage 12. With this structure, pressure fluctuation occurs due to the Karman vortexes alternately generated in both sides of the vortex generating pole 2, and the flows alternately produced in the right and left directions within the detection passage 12 due to this pressure fluctuation are detected as the resistance variation of the thermal flow sensor 13 caused by its temperature variation, thereby measuring the flow velocity or flow rate.
The prior Karman vortex flow meters produce the following problems coming from their above-mentioned structures. That is, in cases where as shown in FIG. 11 the hot wires are directly placed within the duct in which a fluid flows, the dust included in the fluid accumulates on the hot wires in a long time use, and the accumulated dust can hinder the heat radiation of the hot wires, with the result that the electric output of the hot wires decreases to make difficult the detection of the Karman vortexes before long.
Further, in cases where as shown in FIGS. 12 or 13 the holes are made in the vicinity of two vortex separation points on the side surfaces of the vortex generating pole and the detection passage is formed to set up the communication between the holes, the Karman vortexes alternately arising with respect to the detection passage interfere with each other, so that the generation of the Karman vortexes can come into instability.
Moreover, the flow of a fluid (air) taken in an internal combustion engine involves pulsation due to the opening and closing operations of an air intake valve, a throttle valve and other devices. If measuring the period of the Karman vortexes in the flow with a large amplitude of the pulsation, the measurement becomes unstable owing to the effects of the pulsation, with the result that difficulty is encountered to correctly measure the Karman vortexes.